This application pertains to the art of induction heating devices and, more particularly, to an induction strip heater having multiple induction coils arranged in tandem through which the strip being heated passes.
The invention is especially applicable to an induction strip heating line provided with multiple induction coils where two or more high frequency converters are employed as power sources to the coils and where the power sources may not run in synchronism with each other. However, it will be appreciated to those skilled in the art that the invention could be readily adapted to other uses and environments as, for example, where inter-power source energy coupling between adjacent coils is intended to be suppressed.
Induction strip heating lines having multiple induction coils arranged in tandem through which the strip being heated passes are well known in the art. Frequently, two or more high frequency converters are required as power sources. The output frequency of those converters depends upon the resonant frequency of a tank circuit comprised typically of a capacitor bank connected in parallel with the induction coil and its strip load. The firing of rectifier devices (SCRs) in the inverter section of the converters is timed by a feedback voltage from the tank circuit so that the inverter firing is caused to reinforce the natural frequency of the tank circuit. With the necessary physical arrangement of induction coils the currents in one coil will induce voltage in adjacent coils. Since the inverters of each converter are fired in harmony with its own tank circuit voltage, adjacent converters may not run in synchronism with each other. When an induced voltage from another converter circuit appears in the second tank circuit it distorts the wave shape being fed back to the firing control and may cause the inverters to fire at the wrong time creating an unstable condition which prohibits operation of the line. Where power supplies are operating at substantially the same operating frequency, they have a tendency to lock in synchronous operation and prevent independent control of the power supplies. The problem of independently and precisely controlling the power to the coil is the problem that the subject invention seeks primarily to correct.
Various forms and types of devices and assemblies have heretofore been suggested to reduce the power source energy coupling between adjacent coils. A first scheme comprises detuning the tank circuits so that each converter runs at a different frequency. In the heating of thin strips this is not always a commercially viable solution since it is highly desirable to keep the frequency as high as possible to avoid loss of heating efficiency. Another proposal comprises placing adjacent coils much farther apart. This is also not usually practical because a substantial space must be placed between the coils to avoid the coupling problem and this method would thereby disadvantageously increase the amount of space required for the induction heating installation.
The present invention contemplates a new and improved assembly which overcomes the above referred to problems and others to provide a new induction heating assembly which is simple in design, economical to manufacture, readily adaptable to a variety of dimensional characteristics, is rugged and reliable in operation, and which provides improved flexibility and operation in an induction strip heating line.